BOSTON — As it seeks investors, a project off the Massachusetts coast that aims to be the nation's first offshore wind farm must reach fast-approaching benchmarks or risk missing out on hundreds of millions in critical funding.

To qualify for a tax credit that would cover a major portion of its capital costs, Cape Wind either must begin construction by Dec. 31 or prove it's incurred tens of millions of dollars in costs by then.

Also, a $200 million investment — the only one of a specific dollar amount Cape Wind has announced — is conditioned on whether developers can fully finance the rest of the project by year's end.

With less than two months until the deadline, Cape Wind isn't publicly discussing financing efforts. It also has yet to start on-site construction and isn't detailing how it can qualify for the tax credit, only that it expects to.

Even if Cape Wind fails to qualify, spokesman Mark Rodgers said, "We will move this project forward, we will secure financing and we will construct the project."

The 130-turbine, $2.6 billion Cape Wind project was proposed for Nantucket Sound in 2001 and touted as a large, clean energy source near a high-demand coastal area. It's been delayed by a thick bureaucracy and opponents who say the project will ruin the sound and threaten wildlife and maritime traffic.

Cape Wind has sold about three-quarters of its planned power output to two local utilities and aims to be producing power for homes and businesses in Massachusetts by 2015.

First, it must continue to lock down what financing it has and get more of it.

Of two major federal tax subsidies available to renewable energy projects, Rodgers said Cape Wind is pursuing one, an investment tax credit, which could cover a hefty 30 percent of its capital construction costs.

But to qualify for the credit, the project must have begun construction by Dec. 31. Alternatively, Cape Wind can qualify if developers incur 5 percent of the wind farm's cost by year's end.

If the project doesn't qualify for the credit, Cape Wind would be left to fill a huge financing hole. And under its deals with the utilities, failure to obtain the credit would increase the starting price of its power from 20 cents per kilowatt hour to 22.7 cents, with 3.5 percent annual increases.

It's unclear how that would impact the average utility customer's bill. But estimates when the utilities struck their deals (and Cape Wind's starting price was projected at a lower 18.7 cents per kilowatt hour) indicated their average ratepayers would pay about $1 to $1.50 extra per month for Cape Wind's power.

Any further bump in price is sure to inflame critics, who frequently note that Cape Wind's power is far more expensive than other energy sources, including more than double that of land wind.

Though Cape Wind hasn't started erecting turbines in Nantucket Sound, IRS regulations provide other ways to qualify as having begun construction, said Arnold Grant, a tax law expert at Reed Smith, which helps develop renewable energy projects but isn't tied to Cape Wind.

For instance, if an offshore wind farm's turbine supplier is doing significant work off-site, that can count toward having begun construction.

Grant also said meeting the 5 percent costs milestone doesn't even require the company to spend the money by Dec. 31. The delivery of goods from a large equipment contract, for example, can help a company meet the threshold, even the company hasn't paid the contract by year's end.

The bottom line, Grant said, is that companies looking to qualify for the investment tax credit can usually figure it out.

"The rules are out there. You need to satisfy them, but there are different ways of doing it," he said.

Audra Parker of the Alliance to Protect Nantucket Sound, a Cape Wind opponent, said the fact construction hasn't started in the sound speaks to Cape Wind's slow progress, whether it gets the credit or not.

"They can't begin physical construction in Nantucket Sound, so any other means of wasting taxpayer money on Cape Wind would be obtained through a loophole," Parker said.

It may be a steeper climb for Cape Wind to secure more than $2 billion in financing by year's end to meet the conditions of the Danish pension fund PensionDanmark's $200 million investment.

Even if PensionDanmark comes through with a commitment, the project appears well short of the needed financing, said Parker. Cape Wind is pursuing a loan guarantee from the Department of Energy, but it's unknown when a decision will be made.